1. Field of the Invention
The invention relates generally to tunneling magnetoresistance (TMR) devices, and more particularly to a TMR read head with low magnetic noise for use in magnetic recording disk drives.
2. Description of the Related Art
A tunneling magnetoresistance (TMR) device, also called a magnetic tunneling junction (MTJ) device, is comprised of two ferromagnetic layers separated by a thin insulating tunneling barrier layer. The barrier layer is typically made of a metallic oxide, such as alumina (Al2O3), TiO2, or MgO, that is so sufficiently thin that quantum-mechanical tunneling of charge carriers occurs between the two ferromagnetic layers. This quantum-mechanical tunneling process is electron spin dependent, which means that an electrical resistance measured when applying a sense current across the junction depends on the spin-dependent electronic properties of the ferromagnetic and barrier layers, and is a function of the relative orientation of the magnetizations of the two ferromagnetic layers. The magnetization of the first ferromagnetic layer is designed to be pinned, while the magnetization of the second ferromagnetic layer is designed to be free to rotate in response to external magnetic fields. The relative orientation of their magnetizations varies with the external magnetic field, thus resulting in change in the electrical resistance. The TMR device is usable as a memory cell in a nonvolatile magnetic random access memory (MRAM) array, as described in U.S. Pat. No. 5,640,343, and as TMR read head in a magnetic recording disk drive, as described in U.S. Pat. No. 5,729,410.
FIG. 1 illustrates a cross-sectional view of a conventional TMR read head 10. The TMR read head 10 includes a bottom “fixed” or “pinned” ferromagnetic (FM) layer 18, an insulating tunneling barrier layer 20, and a top “free” FM layer 32. The TMR read head 10 has bottom and top nonmagnetic electrodes or leads 12, 14, respectively, with the bottom nonmagnetic electrode 12 being formed on a suitable substrate. The FM layer 18 is called the “pinned” layer because its magnetization is prevented from rotation in the presence of an applied magnetic field in the desired range of interest for the TMR device, i.e., the magnetic field from a recorded region of the magnetic layer in a magnetic recording disk. The magnetization of the pinned FM layer 18 can be fixed or pinned by being formed of a high-coercivity film or by being exchange-coupled to an antiferromagnetic (AF) “pinning” layer. The pinned FM layer 18 may be replaced by an antiparallel (AP) pinned or flux-closure structure, where two ferromagnetic layers are separated by an antiparallel coupling (APC) spacer layer and thus antiparallel-coupled to form a flux closure, as described in U.S. Pat. No. 5,465,185. The magnetization of the free FM layer 32 is free to rotate in the presence of the applied magnetic field in the range of interest. In the absence of the applied magnetic field, the magnetizations of the FM layers 18 and 32 are aligned generally perpendicular in the TMR read head 10. The relative orientation of the magnetizations of the FM layers 18, 32 determines the electrical resistance of the TMR device.
For TMR read heads, thermally-induced magnetic noise (sometimes called “mag-noise”) has become an increasingly relevant problem. The effect of thermal or other energetic excitations on the free FM layer becomes increasingly important as the free layer volume (and therefore its magnetic energy) is reduced. In particular, mag-noise is proportional to the TMR signal. Consequently, for sensors with very high TMR, mag-noise becomes the dominant source of noise and limits the overall signal-to-noise ratio (SNR) of the sensor.
What is needed is a TMR read head with a free ferromagnetic layer with low mag-noise.